Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

Abstract

A series of new bis(pyrazol-3-yl)pyridines (LR) N,N′-disubstituted by 4-functionalized 2,6-dibromophenyl groups have been synthesized to study the effect of a distal substituent on the spin-crossover (SCO) behaviour of the iron(II) complexes [Fe(LR)2](ClO4)2 by variable-temperature magnetometry, NMR spectroscopy, and X-ray diffraction. The SCO-assisting tendency of the substituents with different electronic and steric properties (i.e., the bromine atom and the methyl group) in the para-position of the 2,6-dibromophenyl group is discussed. Together with earlier reported SCO-active iron(II) complexes with N,N′-disubstituted bis(pyrazol-3-yl)pyridines, these new complexes open the way for this family of SCO compounds to emerge as an effective ‘tool’ in revealing structure–function relations, a prerequisite for successful molecular design of switchable materials for future breakthrough applications in sensing, switching, and memory devices.

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N′-Disubstituted Bis(pyrazol-3-yl)pyridines

“…The thermodynamic parameters of the SCO in these solvents are more consistent between the two complexes (Table ) and with other complexes of N , N ′-disubstituted 3-bpp. , The resulting midpoint temperatures of 307 and 214 K for [Fe­( L OH ) 2 ]­(BF 4 ) 2 and [Fe­( L t ‑Bu ) 2 ]­(BF 4 ) 2 mirror the same electronic effect of the substituents in the pyridyl and pyrazol-3-yl moieties on the SCO behavior as identified in DMF but also show its solvent dependence (Figure ). For [Fe­( L OH ) 2 ]­(BF 4 ) 2 , more polar methanol (if judged by Reichardt’s parameters of solvent polarity) shifts the SCO toward higher temperatures, although an inverse trend was previously observed for an iron­(II) complex of unsubstituted 3-bpp, [Fe­(3-bpp) 2 ]­(BF 4 ) 2 .…”

“…As a result, thermodynamic parameters of the SCO in [Fe­( L OH ) 2 ]­(BF 4 ) 2 (Table ) are typical of iron­(II) complexes of N , N ′-disubstituted 3-bpp, , while those in [Fe­( L t ‑Bu ) 2 ]­(BF 4 ) 2 seem underestimated. They, however, still fall into the ranges expected for SCO-active iron­(II) compounds in solutions (Δ H = 4–41 kJ/mol, Δ S = 22–146 J/mol K) .…”

“…The Fe–N bond lengths and the shape of the coordination polyhedra at 120 K (Table ) were typical of the iron­(II) ion in a N6-coordination environment of 3-bpp ligands that adopts the LS state. The latter was also hinted by the distinct red color of the crystals retained upon warming to room temperature, as they should turn yellow in an event of an SCO to the HS state of an iron­(II) complex of 3-bpp. ,− However, the quality of the crystals was too low to perform an X-ray diffraction study for [Fe­( L R ) 2 ]­(BF 4 ) 2 at this temperature. Given a general tendency of N , N ′-disubstituted 3-bpp to produce HS complexes, ,− it may be a sign of a thermally induced SCO that is “blocked” by crystal packing effects. , …”

“…This may be because the Curie law is applicable for ideal spin systems with no contributions from zero-field splitting effects, low-lying excited states, or molecular dynamics that may sometimes occur in iron­(II) complexes. ,,,,, Therefore, the SCO curves (Figure S8 of the Supporting Information and Table ) and appropriate thermodynamic parameters for [Fe­( L OH ) 2 ]­(BF 4 ) 2 and [Fe­( L t ‑Bu ) 2 ]­(BF 4 ) 2 were obtained from the fit with the first-order temperature-dependent Curie constants that account for the above effects . Upon doing so, the mean square error in the best fit for [Fe­( L t ‑Bu ) 2 ]­(BF 4 ) 2 in DMF- d 7 dropped to 0.04 (Table S3 of the Supporting Information) and the thermodynamic parameters (Table ) became more typical of iron­(II) complexes with N , N ′-disubstituted 3-bpp ligands. , …”

“…In our search for SCO complexes of N , N ′-disubstituted bis­(pyrazol-3-yl)­pyridines (3-bpp) ,− devoid of NH groups close to the coordinating nitrogen atoms that preclude − their “truly molecular” design, a family of SCO-active complexes was obtained , by a counterintuitive SCO-assisting ligand design with ortho-functionalized N -phenyl groups (Scheme ). Of them, ligand L with dichlorophenyl N -substituents and hydroxyl groups in the fifth position of the pyrazol-3-yl moiety produced the thermally induced SCO with the highest midpoint temperature of 270 K in a solution …”